MAINTAINERS: update mailing list for devfreq(DVFS).

Devfreq is a power-management framework. Thus linux-pm mailing list is
recommended.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

MAINTAINERS

@@ -2516,7 +2516,7 @@ F:	drivers/usb/dwc3/
 DEVICE FREQUENCY (DEVFREQ)
 M:	MyungJoo Ham <myungjoo.ham@samsung.com>
 M:	Kyungmin Park <kyungmin.park@samsung.com>
-L:	linux-kernel@vger.kernel.org
+L:	linux-pm@vger.kernel.org
 S:	Maintained
 F:	drivers/devfreq/